It will be briefly recalled here that a continuous casting operation consists in pouring a molten metal downwards into a bottomless mould essentially consisting of a metal mould body (made of copper or a copper alloy), generally consisting of assembled plates for the casting of flat products, defining a passage for the cast metal. The walls are vigorously cooled by the circulation of water so as to continuously extract, via the base of this mould, a product that has already solidified on the outside over a few millimetres of thickness. The solidification progresses from the periphery before finally reaching the centreline of the product during is descent downstream of the mould in what is called the “secondary cooling” zone, in which zone the cast product, guided by support and guide rolls (hereafter called support rolls), is sprayed with water in order to extract the heat necessary for its complete solidification. The solidified product thus obtained is then cut to length, then rolled before being shipped to the customer or converted on site into plate products, sheet products, etc.
In the case of flat metal products, and therefore of elongate cross section, commonly called slabs, it has already been known for a long time to effect electromagnetic stirring of the molten metal in the secondary cooling zone of the continuous casting plant.
Schematically, the electromagnetic stirring consists, as is known, in subjecting the slab to one or more moving magnetic fields (that is to say fields in which the maximum intensity moves over time in a defined direction in space), the action of which on the molten metal is therefore manifested by entrainment of the metal identical, in sense and direction, to the displacement of the magnetic field.
In the case of casting flat products, the liquid metal is generally entrained using linearly moving magnetic fields, called travelling fields, undergoing a horizontal translational movement parallel to the broad faces of the product.
The travelling magnetic field is created by a polyphase linear inductor, which is placed as close as possible to the slab so as to maximize the electromagnetic coupling with the latter.
For this purpose, the inductor may be mounted either behind the support rolls, in a solution called a “box-type stirrer”, or inside a support roll, made hollow for this purpose, in the secondary cooling zone, a solution called a “stirrer-roll” or an “in-roll stirrer”.
The two solutions have coexisted on the market since the beginning of the 1980s and have been used right from the start to improve the internal soundness of the cast metal. This is because, thanks to the stirring, the natural crystalline growth of the “dendritic” type from the outside towards and right to near the centreline of the product is interrupted to the benefit of the development of a finer non-oriented solidification structure, called “equiaxed” structure. This thus results in a reduction in the central porosity and a simultaneous reduction in axial macrosegregation (see European Patent 0 097 561). This improvement in the internal soundness was essentially sought for steel grades that are rolled with a low deformation ratio in order to become heavy plate products.
It has been discovered that to achieve, in the secondary cooling zone of a steel slab caster, optimum stirring as regards internal soundness of the product obtained, it is necessary to stir not only in a single localized position but, on the contrary, at least twice over the metallurgical length, that is to say to carry out staged stirring.
It is this that specifically the aforementioned EP 0 097 561 B2 proposes, which patent describes a method for electromagnetically stirring continuously cast steel slabs, in which a plurality of travelling magnetic fields produced by pairs of staged stirrer-rolls are made to act over the metallurgical length, the space between the upper pair and the lower pair being from 1 to 2 metres. Thus, based on a set of four stirrer-rolls in total, the pair of stirrer-rolls closest to the mould is located about 5 to 7 m below the free surface of the liquid metal in the mould and the second pair of stirrer-rolls, located as close as possible to the bottom of the solidification well, is placed at about 4 to 6 m from this bottom. The power supply for the rolls is furthermore regulated so that the magnetic field created by the upper pair travels in the opposite direction to that of the magnetic fields created by the lower pair.
According to that teaching, the stirrer-rolls are thus mounted in the region of the secondary cooling zone, in what are called the “lower segments” of the caster. They are substituted for the support rolls normally provided at these points and therefore have a geometry, especially an outside diameter, identical, or in all cases approximately identical, to that of the adjacent rolls which, in this secondary cooling zone, typically have a diameter of at least 230 mm.
The staged stirring is generally carried out with stirrer-rolls, although in principle is could also be carried out with two box-type stirrers. However, the latter are markedly more expensive as they require about five times more electrical power because of their distance from the surface of the slab, so that staged stirring with box-type inductors would be prohibitively costly.
This electromagnetic stirring technique in the secondary cooling zone, although very widely used throughout the world to improve the quality of heavy plate products, was replaced in the 1990s with a competitive technique called “soft mechanical reduction”. This may in fact be compared to a soft rolling step already in casters, so as to force the solidification fronts on each side of the broad faces of the slab to come together and thus reduce the central porosity and the central segregation more effectively than with electromagnetic stirring.
Consequently, electromagnetic stirring in the secondary cooling zone is practically no longer used at the present time, except in the case of stainless steels and silicon steels, and then for a different metallurgical purpose. This is because there is a problem specific to the continuous casting of these steel grades for which there are often observed, on the products obtained after rolling or drawing, surface defects of the “roping” or “ridging” type, which are manifested by a wavy surface appearance. Such a surface is optically unsatisfactory in the case of stainless steels and, in the case of silicon steels, creates compactness problems in stacks for the production of laminations for transformer or motor yokes.
However, it is already known that this roping and ridging problem may be eliminated if the slab has a solidification structure with a very high fraction, i.e. at least about 50%, of the equiaxed type. Theoretically, it would be possible to obtain such a result by casting the metal with an extremely low level of superheat, but in practice this is impossible in continuous casting and therefore electromagnetic stirring is required in order to rapidly extract this superheat.
Contrary to heavy plate products, for which the porosity and axial segregation must be minimized, the aim here is to maximize the extent of the equiaxed solidification fraction. This is the reason why the stirring must be raised towards the top in the secondary cooling zone in order to be as close as possible to the mould in segment zero of the caster.
It will be recalled that “segment zero” is that segment which receives the cast product directly on leaving the foot rolls of the mould. It defines a particular portion of the metallurgical length that extends over a distance of about 3 to 4 m from the mould exit. This portion, formed by a tight battery of small-diameter (typically around 150 mm) support rolls, is considered to be particularly critical by caster manufacturers. This is particularly so as regards the small spacing between the contact generatrices and the regularity of the mechanical support for the slab, the solidified metal shell of which, which is still relatively thin, runs the risk of bulging in the space between two successive mechanical support rolls as it is subjected to an already high ferrostatic pressure at this point.
It is therefore in order not to locally modify the regularity and the small spacing of the rolls supporting slab in segment zero that electromagnetic stirring by means of box-type inductors placed behind these small support rolls is proposed therein, whereas fitting stirrer-rolls of substantially larger diameter would involve a discontinuity in the spacing of the support rolls.
However, box-type stirring requires that any metal structure present between the inductor and the slab to be made of non-magnetic steel so as not to form a screen to the acting magnetic field. This involves modifying segment zero when the aim is to introduce stirring in existing casters, or a specially designed segment zero, which is therefore more expensive, when producing new casters. Furthermore, despite the small diameter of around 150 mm of the support rolls in segment zero, the distance between the slab and the box-type inductor cannot be reduced to below 270 to 250 mm because of the mechanical structure behind the support rolls, which structure supports the intermediate bearings for these rolls. As already explained above, this imposed distance between inductor and cast product greatly degrades the electromagnetic coupling between the two, and as a compensation requires a great increase in electrical power.
The state of the art in the case of stainless steels and silicon steels is therefore characterized by: (i) localized stirring in segment zero of the caster in order to obtain an equiaxed zone width of about 50% or more of the thickness of the slab; (ii) use of box-type inductors behind the small support rolls, in order not to locally modify the diameter and the position of said rolls; (iii) consequently, limiting stirring to a single non-staged stirring operation for cost reasons, although staged stirring gives better results; and (iv) impossibility of varying the position of the stirrer for a given segment zero.